For testing the EF-6 amplifier, a set of AKG K701 headphones was used for the test load. As they are a very demanding load that requires a dedicated amplifier to get any bass out of them, they push the amplifier as hard as almost any non-electrostatic headphones will.

On the 1 kHz test tone the 2nd and 3rd harmonics are both very low, giving us over 80 dB of headroom and a low noise floor that has a slight right starting around 45 kHz. The main issue seen is a 60 Hz tone which seems to indicate some noise from the power supply getting into the signal.

With a 10 kHz tone we see very similar results, with close to 80 dB of room above the 2nd harmonic though once again that 60 Hz tone is there around 70 dB below the fundamental tone.

The 19 kHz and 20 kHz IMD test shows that the sidebands are 70 dB below the primary tones and drop off quickly. There is also a peak at 31 kHz that is present in all the other charts as well. It is low and likely not audible at all, but still present.

With the 60 Hz + 7000 Hz IMD test, the 60Hz spike from the power supply isn't an issue since it has a tone there as well. We once again have over 80 dB of headroom here, and a pretty low IMD number for an amp being driven to this level.

If we zoom in very close on the frequency response we see there is a very slight rise in output from 20 Hz to 20,000 Hz, but no more than 1 dB across the whole range, and then a slight roll-off after it peaks at 45 kHz, before a steep fall off after 80 kHz.

Across the whole frequency range the THD+N measures in at right around 0.07%.

I did run some additional testing with lower power loads and less demanding loads from headphones on the EF-6 and this caused improvements in all measurements across the board, allowing me to reach the 95 dB of SNR that HiFiMan has specified. These tests indicate something close to a worst case scenario, and show that the EF-6 has good performance across the board even then.